In telecommunication networks, a subscriber device is connected to other subscriber devices using various connection techniques. For example, over short distances, two-wire lines may be employed. For longer distances, four-wire lines may be used. In addition, hybrid circuits are used to connect two-wire lines to four-wire lines.
Voice messages are transmitted through the network. For instance, if the subscriber device is a telephone, a voice message may be transmitted from a telephone, over a two-wire line to a first hybrid circuit, and then over a four-wire line to a second hybrid circuit. The second hybrid circuit may be connected to a second subscriber device.
In addition to voice messages, telecommunication networks also transmit data signals. Many of the data signals are narrow-band. For example, narrow-band data signals generated by V.21 modems are transmitted over telecommunication networks. Although some data signals can not be classified as narrow-band signals, these signals are modulated, and, therefore, their energy centers around a carrier frequency.
One consequence of using hybrid circuits to couple different types of connections together is the creation of echoes. That is, speech may be reflected by a hybrid circuit back to the speaker, causing the speaker to hear their own voice. Echo cancellers are used to minimize or eliminate the effects of these echoes.
Echo cancellers may use adaptive filters. An adaptive filter, using a filtering algorithm, produces a mathematical model of echo characteristics, which is used to generate an echo estimate. For example, if a first subscriber device sends a signal to a second subscriber device, an echo may be created. A return signal is formed, which comprises a signal sent from the second device together with the echo. When echo cancellation is performed, the echo canceller subtracts the echo estimate (created by the adaptive filter) from the return signal. Thus, the signal received by the first device should be echo-free or substantially echo-free.
However, the filtering algorithms used in adaptive filters have inherent limitations. For example, an adaptive filter, which uses the least mean square (LMS) algorithm in the adaptive filter, may diverge for narrow-band data signals. Specifically, although the echo may be cancelled, the adaptive filter will not converge the true echo path. In fact, the echo canceller will diverge to a filter related to the narrow-band frequency.
In other situations, the adaptive filter will change with the change of carrier frequency and re-converge to a different adaptive filter. The new adaptive filter will not converge to the true echo path. As a result, the echo canceller will generate an image echo when the carrier frequency changes.
For example, in the call establishment phase for a fax transmission, data is transmitted in the form of a tone with a frequency of approximately 2100 Hz and a duration of 2.6 to 4 seconds. Then, additional data is transmitted that is a modulated signal with a carrier frequency of 1750 Hz. The echo canceller, in this case, generates an image echo caused by the previously converged adaptive filter. The amplitude of the image echo depends upon the frequencies, tone amplitudes, and the adaptation constant. The image echo may be a higher value than the true echo. As a result, there may be bit errors introduced in the data.
In still other situations, the echo canceller may introduce errors in data signals. When both far-end and near-end inputs are narrow-band signals, for example, in a double-talk situation or during full duplex transmission, the adaptive filter will converge to a filter related to the two carrier frequencies.
For example, when a V.21 modem is used in full duplex mode, the caller may use a frequency of 1080 Hz and the called station may use a carrier frequency of 1750 Hz. Double-talk will occur since the caller and called station transmit at the same time. In this situation, the echo canceller will cancel some portions of the data signal since the adaptive filter converges to a filter related to the carrier frequencies. Since the near-end signals may be cancelled to some extent, the echo canceller may think that it is not in a double-talk situation. The call may be either dropped or bit errors produced as a result of the processing.